Data processing systems frequently include data input devices employing slips bearing coded magnetic information. Examples of these slips are bank checks, post office checks or credit cards. The magnetic coded information generally includes a series of alpha-numeric characters printed on the slips; the characters are a succession of letters of the alphabet, figures, punctuation marks, etc. If the slip is a check, the alpha-numeric characters indicate the check number and/or the account number of the drawer. Each character is formed by a group of bars composed of magnetic ink. The number of bars, the distance between them, and the relative disposition of the bars are individual to each character and conform to known codes, such as the CMC7 code. For simplicity, the present invention is described as it applies to a check reading device but the invention also applies to any system in which it is necessary to detect a magnetic field.
A check reader converts the coded magnetic information represented by the characters printed on the check into a series of electrical signals. The signals are changed by electronic shaping circuits into a series of square wave electrical pulses which are fed to electronic circuits that recognize the characters printed on the check. When the characters corresponding to this series of square wave electrical pulses, which correspond in turn to the printed characters, have been identified, a calculating unit in a data processing system of which the check reader is a part performs operations relating to the check; exemplary of the operations are debiting, crediting and updating the account of the drawer.
To enable the invention to be better understood, the following facts about magnetism are reviewed:
To magnetize a magnetic material in which the magnetic induction is very weak, the material is initially subjected to a positive magnetic field having sufficient strength to saturate the material; that is, the magnetic induction in the material reaches a limiting value B.sub.s when the strength of the magnetic field reaches a certain value H.sub.s. The magnetic field is then removed. There remains a non-zero magnetic induction (+Mr) termed the residual induction, having an amplitude characteristic of the material. A magnetized magnetic material generates a leakage magnetic field H in the immediate vicinity of its surface. The magnetic flux of a magnetic field H through an area S is equal to the product of the strength of field H multiplied by the size of the areas.
Check readers generally comprise a magnetizing device and a magnetic transducer. The magnetizing device magnetizes character bars printed on the check to render the value and sense of the magnetic induction in all the bars identical. This is necessary because printing the characters on the check causes the induction of the bars to be zero or the value and polarity of the magnetic induction to vary from one bar to the next throughout the bars. Thus, the magnetic induction in the bars equals the residual induction of the magnetic ink when the bars are no longer subject to the magnetic field of the magnetizing device.
The magnetic transducer device derives an electric signal in response to the magnetic leakage field set up by the bars magnetized by the magnetizing device. The signal is supplied to the previously mentioned electronic shaping circuits. In other words, the magnetic transducer device detects the presence of the bars.
The check is positioned in and moved by a mechanical check transporting device so that all the bars pass in succession in front of the magnetizing and transducer devices, which are in close proximity to each other.
Existing magnetic transducer devices employing at least one magnetoresistor are simple, inexpensive and highly reliable in detecting the presence of bars, while being insensitive to the speed of the bars relative to the transducer device. Magnetoresistors are electrical resistors having variable resistance values as a function of the magnetic field to which they are subjected. Typically, magnetoresistors are thin films or layers of very shallow depth (being a few hundred Angstroms to a few microns thick) deposited on an insulating substrate. Assume that a resistor R is connected to the terminals of a generator having an output current I, whereby a voltage V is developed across the magnetoresistor. When a bar passes in front of the magnetoresistor, the flux, H, of the bar magnetic leakage field causes a change, .DELTA.R, in the value of R with a resulting voltage change .DELTA.V. The equal ratios .DELTA.V/V and .DELTA.R/R are termed the coefficient of magnetoresistance, which is usually on the order of 0.5 to 2% and is very often negative.
An electric signal corresponding to the value of .DELTA.R is amplified and supplied to the aforementioned shaping circuits. This signal is unaffected by the speed of the bars relative to the magnetoresistor.
Presently, magnetic transducer devices employing magnetoresistors usually employ two or three magnetoresistors that detect the presence of several bars deposited at a distance or pitch p from one another, on a single insulating substrate. The substrate is moved relative to the magnetoresistors so the bars pass in turn in front of each of the magnetoresistors. The distance p depends on the width of the bars and the maximum and minimum spacing between them. Such a device is described in an article entitled "Dual Stripe Magnetoresistive Read Heads for Speed Insensitive Tape Readers" by G. E. Moore, Jr. and Lijote, published in the "IEEE Transactions on Magnetics", vol. 12, number 6, November 1976.
Since the signal derived from the magnetoresistors is independent of the speed of the characters relative to the magnetoresistors it is possible in theory to detect the presence of bars by stopping a bar carrying a check in front of the magnetoresistors, i.e., by maintaining the check stationary in front of the magnetoresistors. Existing magnetoresistor magnetic transducer devices do not use this advantage because they contain a relatively small number of magnetoresistors; the number of magnetoresistors is very much smaller than the number of bars in the characters printed on the checks.
To exploit this advantage, it is necessary to use a very large number of equidistantly positioned magnetoresistive elements (a few hundred or even a few thousand). The pitch p must depend on the width of the bars and the maximum and minimum spacing between them. By placing the check in front of the magnetic transducer device it is possible, in response to the amplitude of the output signal developed across each magnetoresistive element, to detect whether or not a bar is in front of the element. From the output signals, the coded magnetic information on the check can be reconstituted without relative movement between the checks and the magnetic transducer device.
Magnetic transducer devices employing a large number of magnetoresistors are disadvantageous because it is necessary to detect resistance changes of 0.5 to 1%. Such small resistance changes must be detected because hundreds or even thousands of magnetoresistive elements are required in the transducer device. Manufacturing such a large number of these elements to standards of consistency is unrealistic using existing technology.